Integrated utilization of silicon tetrafluoride and zirconium dioxide

Article abstract of DOI:10.1023/B:RJAC.0000022435.36383.9d

The reaction of silicon tetrafluoride with zirconium dioxide was studied. A technological scheme was suggested for utilization of silicon tetrafluoride and reprocessing of spent zirconium dioxide to obtain zircon and zirconium tetrafluoride.

Full text of DOI:10.1023/B:RJAC.0000022435.36383.9d

Russian Journal of Applied Chemistry, Vol. 76, No. 12, 2003, pp. 1900 1903. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 12, 2003,
pp. 1952 1955.
Original Russian Text Copyright
2003 by Guzeev, D’yachenko, Grishkov.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Integrated Utilization of Silicon Tetrafluoride
and Zirconium Dioxide
V. V. Guzeev, A. N. D’yachenko, and V. N. Grishkov
Tomsk Polytechnical University, Tomsk, Russia
Institute of Strength Physics and Materials Science, Siberian Division, Russian Academy of Sciences, Tomsk, Russia
Received March 19, 2003
Abstract The reaction of silicon tetrafluoride with zirconium dioxide was studied. A technological scheme
was suggested for utilization of silicon tetrafluoride and reprocessing of spent zirconium dioxide to obtain
zircon and zirconium tetrafluoride.
At present, numerous industrial enterprises en-
counter difficulties with utilization of silicon tetra-
fluoride, a large-tonnage by-product formed in pro-
duction of superphosphate, aluminum, and hydrogen
fluoride and in some other industries [1]. Commonly,
utilization of SiF4 includes a stage of hydrolysis, with
subsequent processing of H2SiF6 and SiO2 [2].
The resulting gas was bubbled through sulfuric
acid 3 to remove trace amounts of moisture and then
passed through a column with fluidized bed of finely
dispersed SiO2 4 to remove HF admixtures.
Purified SiF4 was fed into a double-zone quartz re-
actor 5. The first zone, 6, was heated, with temperature
controlled and measured with a Chromel Alumel ther-
mocouple. A quartz boat with weighed sample of ZrO2,
7, was placed in a quartz tube. Sets of experiments were
carried out at 700, 800, 900, and 1000 C and varied
reaction duration. When a prescribed time elapsed,
the sample was removed and analyzed. The qualitative
and quantitative compositions of samples were deter-
mined by chemical, gravimetric, and X-ray analyses.
It was found in studying certain physicochemical
properties of SiF4 that this gas is a good selective
fluorinating agent. In particular, the reaction of silicon
tetrafluoride with zirconium dioxide was studied.
Processing of waste ceramics based on zirconium
dioxide is hindered by its low chemical activity.
Ceramic ZrO2 is insoluble in pure mineral acids,
although it reacts slowly with a mixture of sulfuric
and hydrofluoric acids. Usually, high-temperature
techniques of fusion with oxides or hydroxides of
alkaline-earth metals are to be used in processing of
ZrO2 wastes [3].
The reaction of silicon tetrafluoride with zirconium
dioxide is known [4]; however, no related exper-
imental data have been published, and the reaction is
not used in technological practice. A detailed study
of the reaction of SiF4 and ZrO2 will make it possible
to develop a technology for utilization of SiF4 and
processing of ZrO2.
We carried out a quantitative study of the reaction
on a laboratory quartz installation (Fig. 1).
Fig. 1. Schematic of laboratory installation: (1) Flask for
synthesis of silicon tetrafluoride, (2) adjustable heating
element, (3) two-necked flask with sulfuric acid, (4) col-
umn packed with silicon dioxide, (5) double-zone quartz
reactor, (6) hot zone of the quartz reactor with heating
Silicon tetrafluoride was formed in flask 1 with
heater 2 by the reaction
element, (7) boat with weighed ZrO sample, (8) cold
2
SiO2 + 2MgF2 + 2H2SO4 = SiF4 + 2MgSO4 + 2H2O.
zone of the reactor (desublimator), (9) absorber with NH OH.
4
1070-4272/03/7612-1900 $25.00 2003 MAIK Nauka/Interperiodica

INTEGRATED UTILIZATION OF SILICON TETRAFLUORIDE AND ZIRCONIUM DIOXIDE
1901
The forming zirconium tetrafluoride sublimated at
temperatures of 900 C and above and was deposited
onto a cooled surface of the quartz tube. Its composi-
tion was determined by chemical analysis.
The remaining sample was treated with concen-
trated sulfuric acid (dried to smoke evolution), and the
dry remainder was treated with water to remove ZrO2
and ZrF2, which were converted into soluble zirconi-
um(IV) sulfate under the action of H2SO4. A light
beige crystalline powder remained in the boat.
An X-ray structure analysis was carried out on
a DRON-3M diffractometer with filtered FeK irradi-
Fig. 2. X-ray diffraction pattern of (a) light beige crystal-
ation. A sample was mixed with a nitrocellulose
lacquer as a binder and compacted in a standard fused-
silica cuvette. With the cuvette rotated around the axis
perpendicular to its surface, reflection intensity fluc-
tuations of about 30% were observed, which points to
the presence of a texture in the compacted powder.
This results from the specific shape of powder par-
ticles. To avoid broadened reflections and intensity
fluctuations upon rotation, further experiments were
carried out with an immobile sample. An X-ray dif-
fraction pattern of a washed light beige crystalline
precipitate is shown in Fig. 2a. An X-ray diffraction
pattern of ZrSiO4 [5] is shown as reference in Fig. 2b.
line precipitate and (b) ZrSiO [5]. <x> denotes, presum-
4
ably, trace amounts of SiO admixture. (2 ) Bragg angle;
2
the same for Fig. 4.
It follows from the X-ray diffraction patterns that
the sample being analyzed mainly contains ZrSiO4
with typical tetragonal structure. The lattice constants
of ZrSiO4 obtained by the technique described above
were determined from reflections (512) and (044):
a = 6.592, c = 5.989 . The error in determining a
and c did not exceed 0.002 . A similar calculation
based on the same reflections from the reference gives
the values: a = 6.588, c = 5.989 , and the tabulated
data for ZrSiO4 [5] are as follows: a = 6.593 and c =
5.942 . Hence the tabulated and experimental values
of parameter a coincide within the limits of the error.
The interplanar spacings corresponding to hkl re-
flections from the sample under study and the refer-
ence and their relative intensities are listed in Table 1.
Table 1. Reflection indexes hkl and interplanar spacings in X-ray diffraction patterns of crystalline deposit, dexp , and
standard compound ZrSiO4 [5], dtabl
Exper-
iment
no.
dexp
dtabl
Exper-
iment
no.
dexp
dtabl
(hkl)
(hkl)
(J)
(J)
1
2
3
4
5
6
7
8
9
10
11
12
13
(011)
4.437 (73)
4.07 (9)
3.85 (4)
4.434 (45)
14
(123)
(004)
(141)
(331)
(240)
(332)
(024)
(510)
(511)
(224)
(512)
(440)
(044)
1.684 (18)
1.651 (14)
15
16
17
18
19
20
21
22
23
24
25
1.545 (1)
1.547 (4)
1.495 (3)
1.477 (8)
1.381 (10)
1.362 (5)
1.290 (5)
1.259 (8)
1.248 (3)
1.1863 (11)
1.1672 (2)
1.1079 (5)
(020)
(121)
(112)
3.301 (61)
2.649 (1)
2.521 (100)
2.49 (4)
2.333 (10)
2.216 (7)
2.067 (36)
1.907 (25)
1.751 (16)
1.712 (41)
3.302 (100)
2.650 (7)
2.518 (45)
1.495 (27)
1.477 (18)
1.380 (70)
1.362 (20)
1.289 (11)
1.259 (25)
(220)
(022)
(031)
(013)
(231)
(132)
2.336 (10)
2.217 (8)
2.066 (20)
1.908 (14)
1.751 (11)
1.712 (40)
1.1869 (15)
1.1082 (23)
*
Relative reflection intensities are given in parentheses after the values of d. Presumably, weak reflection nos. 2, 3, and 7
belong to SiO .
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 12 2003

1902
GUZEEV et al.
of the compound (in particular, with different admix-
tures).
Electron micrographs of synthetic zircon crystals
obtained by the above-described technique and of
zircon crystals from the Tugansk deposit (Tomsk
Province) are shown in Fig. 3. Unlike natural mineral
zircon, synthetic zircon has a well-pronounced crystal
texture.
In addition to the reflections of ZrSiO4, the X-ray
diffraction pattern of the experimental sample contains
weak reflections (Table 1, nos. 2, 3, and 7) unusual
for this compound. Most likely they belong to SiO2,
but a closer examination is required. Thus, the light
beige crystals are mostly composed of zircon ZrSiO4.
Fig. 3. SEM micrographs of (a) zircon from Tugansk de-
posit and (b) synthetic zircon.
The reaction of ZrO2 and SiF4 is likely to be de-
scribed by the following equation:
2ZrO2 + SiF4 = ZrSiO4 + ZrF4.
X-ray diffraction patterns of a sample of zirconium
tetrafluoride sublimated at 950 C and then deposited
onto the surface of a cooled quartz tube are free of
Bragg reflections from its , , and crystalline mod-
ifications, but show intense diffuse scattering (Fig. 4).
Fig. 4. X-ray diffraction pattern of a ZrF sample desub-
limated on a cooled surface. Weak reflections of a crystal-
line admixture phase are indicated.
4
The following reaction mechanism can be sug-
gested: an exchange reaction yielding ZrF4 and ZrO2
occurs in the first stage; then, as temperature increases
and SiO2 is accumulated, SiO2 is sintered with un-
reacted zirconium dioxide. A part of SiO2 formed is
carried away as a finely dispersed solid phase by
a flow of sublimated zirconium tetrafluoride and is
deposited together with it. It is possible that, at lower
temperatures, the reaction proceeds by a purely ex-
change mechanism to give only silicon dioxide and
zirconium tetrafluoride. Kinetic hindrances caused by
the formation of a zirconium tetrafluoride film on
zirconium dioxide particles give no way of performing
the process at a sufficient rate at temperatures lower
than the sublimation temperature of zirconium tetra-
fluoride.
A minor difference in the values of the parameter c
may be due to specific features of the chemical com-
position, associated with different synthesis conditions
Table 2. Thermodynamic parameters of two stages of
the reaction between ZrF4 and SiO2
H
G
T, K
log Kp
Kp
1
kJ mol
ZrO2 + SiF4 = ZrF4 + ZrO2
298
400
106.6
104.5
100.3
94.9
51.0
32.2
2.5
36.4
58.9
99.9
9
109
1.6 104
0.6
4.2
0.2
2.4
3.1
4.4
600
800
1000
1200
We carried out a thermodynamic analysis of the
reactions (Table 2), using the standard technique [6]
and taking into account the high-temperature compo-
nents of the enthalpy and entropy. The analysis de-
monstrated that the reactions cannot proceed in the
equilibrium system at temperatures higher than 500 K.
3
4
5
4
8
4
10
10
10
74.4
69.8
2ZrO2 + SiF4 = ZrF4 + ZrSiO4
298
400
600
800
1000
1200
125.0
122.9
119.1
114.1
109.1
104.1
67.3
47.65
10.45
25.5
59.4
92.0
11.8
6.2
0.9
1.6
3.1
4.0
6
1011
1.6 106
8
It follows from the calculations that the equilibrium
constant of the reactions decreases as temperature
increases, and above 600 K the thermodynamic equi-
librium is shifted in the reverse direction, i.e., the re-
actions cannot proceed.
2
3
2.5 10
1.3 10
10
4
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 12 2003

INTEGRATED UTILIZATION OF SILICON TETRAFLUORIDE AND ZIRCONIUM DIOXIDE
1903
Fig. 5. Kinetics of the process at various temperatures.
( ) Degree of conversion and ( ) time. Temperature ( C):
(1) 700, (2) 800, (3) 900, (4) 1000.
Fig. 6. Plug-flow reactor for utilization of SiF . (1) Col-
4
umn heated to 900 C, (2) desublimator for collecting ZrF ,
4
and (3) reaction front.
The disagreement between the thermodynamic cal-
culations and the actually observed process may be
due to a nonequilibrium state of the system, whereas
the classical thermodynamic approaches are only ap-
plicable to equilibrium processes. In fact, zirconium
tetrafluoride formed at temperatures higher than 900 C
sublimates and leaves the reaction zone (being de-
sublimated on a cold wall reactor), i.e., the reverse
reaction becomes unfeasible and the overall reaction
occurs even at negligibly small equilibrium con-
stants.
CONCLUSIONS
(1) The results obtained in studying the reaction
2ZrO2 + SiF4 = ZrSiO4 + ZrF4 enable several pro-
cesses: (i) utilization of SiF4, (ii) fluorination of ZrO2
without using F2 or HF, and (iii) production of high-
purity ZrSiO4 for manufacture of Bakor-type mate-
rials.
(2) The data obtained make it possible to develop
a technology for utilization of silicon tetrafluoride
and processing of waste ceramic zirconium dioxide.
The results of kinetic experiments at 700, 800, 900,
and 1000 C are shown graphically in Fig. 5. The de-
gree of conversion was calculated from the weight of
zircon formed. The experimental data allow us to sug-
gest two technological processes: (i) utilization of
SiF4 and (ii) that of ZrO2.
REFERENCES
1. Galkin, N.P. and Krutikov, A.B., Tekhnologiya ftora
(Technlogy of Fluorine), Moscow: Atomizdat, 1968.
2. Galkin, N.P., Zaitsev, V.A., and Seregin, M.B., Ulav-
livanie i pererabotka ftorosoderzhashchikh gazov (Re-
covery and Processing of Fluorine-containing Gases),
Moscow: Atomizdat, 1975.
3. Korovin, S.E., Drobot, D.V., and Fedorov, P.I., Redkie
i rasseyannye elementy, khimija i tekhnologija (Rare
and Scattered Elements, Chemistry and Technology),
Moscow: Mosk. Inst. Stali Splavov, 1999, vol. 1.
4. Ryss, I.G., Khimija ftora i ego neorganicheskikh so-
edinenii (Chemistry of Fluorine and its Inorganic Com-
pounds), Moscow: Goskhimizdat, 1956.
5. Powder diffraction file. Alphabetical Index of Inorganic
Compounds 1977. International Centre for Diffraction
Data, 1601 Park Lane, Swarthmore, Pennsylvania
19081, USA.
An integrated utilization of SiF4 and ZrO2 can
also be organized. A schematic of the suggested plug-
flow apparatus for utilization of silicon tetrafluoride is
shown in Fig. 6. The apparatus consists of two parts:
a column 1 with granular zirconium dioxide, heated to
900 C, and a cooled desublimator 2 for collection of
zirconium tetrafluoride. Silicon tetrafluoride fed into
the lower part of the column will react with the excess
amount of zirconium dioxide to give zircon and zir-
conium tetrafluoride.
The completion of the process upon arrival of
the reaction front 3 at the upper part of the column 1
can be judged from the appearance of silicon tetra-
fluoride in the vacuum system.
6. Kireev, V.A., Metody prakticheskikh raschetov v ter-
modinamike khimicheskikh reaktsii (Methods for Prac-
tical Calculations of the Thermodynamics of Chemical
Reactions), Moscow: Khimiya, 1975.
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